The invention relates to a short-circuit-proof transistor output stage, especially ignition output stage for motor vehicles having a power transistor connected in series to a load, and having a blocking device for turning off the power transistor in the event of a short circuit in the load.
In ignition systems for motor vehicles, an ignition transformer (ignition coil) is turned on and off by a power Darlington pair in order to generate an ignition spark. This power Darlington pair connected in the primary circuit of the ignition coil switches coil currents between 5 and 15 amperes. In rare cases, the ignition transformer can be damaged by salt water, dirt, high voltage discharges or excess temperature, so that a primary-side short circuit is produced. Here, due to the battery voltage, the saturation voltage of the transistor and cable resistances give rise to currents of between 20 and 30 amperes, which destroy the output stage of the power Darlington pair, and thus the ignition system.
In order to prevent such a destruction in the event of a short circuit, it is known to turn off the output stage upon overshooting of an upper current threshold. The known arrangement has, however, the disadvantage that despite the short circuit of the ignition coil the power Darlington pair always turns on again upon undershooting of this current threshold. Since this switching operation occurs at very high currents and also high frequencies, the output stage can nevertheless be damaged.
A further known method consists in bonding a temperature sensor to the chip, or integrating it into the output stage. The disadvantage of these sensors resides in their inertia. The output stage can already be destroyed when the excess temperature is displayed and the Darlington pair is turned off.